Optical amplifier with pump light source control for Raman amplification

ABSTRACT

A Raman amplifier for amplifying a wavelength division multiplexed (WDM) light including signal lights wavelength division multiplexed together. The amplifier includes an optical amplifying medium and a controller. The optical amplifying medium uses Raman amplification to amplify the WDM light in accordance with multiplexed pump lights of different wavelengths traveling through the optical amplifying medium. The WDM light is amplified in a wavelength band divided into a plurality of individual wavelength bands. The controller controls power of each pump light based on a wavelength characteristic of gain generated in the optical amplifying medium in the individual wavelength bands.

This application is a divisional of application Ser. No. 10/624,568,filed Jul. 23, 2003, now allowed, which is a divisional of applicationSer. No. 09/693,838, filed Oct. 23, 2000, now U.S. Pat. No. 6,624,926.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on, and claims priority to, Japaneseapplication number 2000-255291, filed Aug. 25, 2000, in Japan, and whichis incorporated herein by reference.

This application is related to U.S. application Ser. No. 09/531,015,filed Mar. 20, 2000, and which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a Raman amplifier for amplifying asignal light in an optical communication system. More particularly, thepresent invention relates to a Raman amplifier for amplifying wavelengthdivision multiplexed signal lights.

2. Description of the Related Art

Almost all optical amplifiers used in current optical communicationsystems are rare-earth doped optical fiber amplifiers. Particularly,erbium (Er) doped optical fiber amplifiers (EDFA) are commonly used.

Moreover, with wavelength division multiplexing (WDM) opticalcommunication systems, a plurality of signal lights at differentwavelengths are multiplexed together and then transmitted through asingle optical fiber. Since an EDFA has a relatively wide gain band, WDMoptical communication systems use EDFAs to amplify the multiplexedsignal lights. Therefore, with WDM optical communication systems usingEDFAs, the transmission capacity of an optical fiber can be greatlyincreased.

Such WDM optical communication systems using EDFAs are extremely costeffective, since they can be applied to previously laid optical fibertransmission line to greatly increase the transmission capacity of theoptical fiber transmission line. Moreover, an optical fiber transmissionlines has virtually no limitation on bit rate since EDFAs can easily beupgraded in the future, as developments in optical amplifier technologyoccur.

Transmission loss of an optical fiber transmission line is small (about0.3 dB/km or less) in the wavelength band of 1450 nm to 1650 nm, but thepractical amplifying wavelength band of an EDFA is in a range of 1530 nmto 1610 nm. Thus, an EDFA is only effective for amplifying signals in aportion of the wavelength band of 1450 nm to 1650 nm.

In a WDM optical communication system, a predetermined transmissioncharacteristic may be obtained by suppressing fluctuation of opticalpower among each channel to 1 dB or less in each optical repeating stagebecause the upper limit of optical power is caused by a non-lineareffect and the lower limit by a receiving signal-to-noise ratio (SNR).

Here, a transmission loss wavelength characteristic of the transmissionline and a dispersion compensation fiber or the like forming the WDMoptical communication system must be reduced.

In a WDM optical communication system, the wavelength characteristic oftransmission loss in a transmission line due to the induced Ramanscattering provides the maximum influence on the wavelengthcharacteristic of the signal light.

A key component of current WDM transmission systems is an EDFA that canamplify wavelength division multiplexed signal lights at the same time.For further improvement, such as increase of transmission capacity andrealization of ultra-long distance transmission, it would be desirableto provide an optical amplifier which can amplify different wavelengthbands than a conventional EDFA, while also providing the favorablecharacteristics of an EDFA.

In view of expanding the wavelength band of an optical amplifier toincrease the transmission capacity of optical fibers, attention is beingdirected to a Raman amplifier.

A Raman amplifier can amplify the Stokes-shifted frequency that isshifted as much as the Raman shift of the amplifying medium from thefrequency of a pump light. Therefore, a signal light can be amplified ata desired frequency with a pump light source producing a pump light of adesired wavelength.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a Ramanamplifier for use in a WDM optical communication system.

More specifically, it is an object of the present invention to provide acontrol algorithm for a Raman amplifier using multiple pump lightwavelengths or pump sources to attain a flat wavelength band over a wideband range.

It is also an object of the present invention to provide a controlalgorithm for a Raman amplifier that allows the amplifier to easilyrealize constant output power control, constant gain control andwavelength characteristic flattening control.

Additional objects and advantages of the invention will be set forth inpart in the description which follows, and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

The foregoing objects of the present invention are achieved by providingan optical amplifier including (a) an optical amplifying medium to Ramanamplify a wavelength division multiplex (WDM) light including signallights wavelength division multiplexed together; (b) pump light sourcesgenerating pump lights of different wavelengths; (c) a first opticalmultiplexer multiplexing the pump lights together; (d) a second opticalmultiplexer multiplexing the WDM light with the multiplexed pump lights;(e) a detector dividing the amplified WDM light into wavelength bandsand detecting a power in each wavelength band; and (f) a pump lightcontroller controlling power of each pump light based on a wavelengthcharacteristic of gain generated in the optical amplifying medium foreach wavelength bands, in accordance with the powers detected by thedetector.

Objects of the present invention are also achieved by providing anoptical amplifier including (a) an optical amplifying medium to Ramanamplify a wavelength division multiplex (WDM) light including signallights wavelength division multiplexed together; (b) pump light sourcesgenerating pump lights of different wavelengths; (c) a first opticalmultiplexer multiplexing the pump lights together; (d) a second opticalmultiplexer multiplexing the WDM light with the multiplexed pump lights;(e) an input detector detecting power of the WDM light before beingamplified by the optical amplifying medium; (f) an output detectordetecting power of the amplified WDM light; and (g) a pump lightcontroller controlling powers of the pump lights based on the powerdetected by the input detector and the power detected by the outputdetector.

Moreover, objects of the present invention are achieved by providing anoptical amplifier including (a) an optical amplifying medium to Ramanamplify a wavelength division multiplex (WDM) light including signallights wavelength division multiplexed together; (b) pump light sourcesgenerating pump lights of different wavelengths; (c) a first opticalmultiplexer multiplexing the pump lights together; (d) a second opticalmultiplexer multiplexing the WDM light with the multiplexed pump lights;(e) a decoupler decoupling a portion of the amplified WDM light; (f) adetector dividing the decoupled portion into wavelength bands anddetecting a power in each wavelength band; and (g) a pump lightcontroller controlling power of each pump light based on a wavelengthcharacteristic of gain generated in the optical amplifying medium foreach wavelength bands, in accordance with the powers detected by thedetector.

Further, objects of the present invention are achieved by providing anoptical amplifier including (a) an optical amplifying medium to Ramanamplify a wavelength division multiplex (WDM) light including signallights wavelength division multiplexed together; (b) pump light sourcesgenerating pump lights of different wavelengths; (c) a first opticalmultiplexer multiplexing the pump lights together; (d) a second opticalmultiplexer multiplexing the WDM light with the multiplexed pump lights;(e) an input detector dividing the WDM light before being amplified inthe optical amplifying medium into wavelength bands, and detecting thepower in each wavelength band; (f) an output detector dividing theamplified WDM light into the same wavelength bands as the inputdetector, and detecting the power in each wavelength band; and (g) apump light controller controlling powers of the pump lights based on thepowers detected by the input detector and the powers detected by theoutput detector

In addition, objects of the present invention are achieved by providingan optical amplifier for amplifying a wavelength division multiplexed(WDM) light including signal lights wavelength division multiplexedtogether, the amplifier including (a) an optical amplifying medium toRaman amplify the WDM light in accordance with multiplexed pump lightsof different wavelengths traveling through the optical amplifyingmedium, the WDM light being amplified in a wavelength band divided intoa plurality of individual wavelength bands; and (b) a controllercontrolling power of each pump light based on a wavelengthcharacteristic of gain generated in the optical amplifying medium in theindividual wavelength bands.

Objects of the present invention are also achieved by providing anoptical amplifier for amplifying a wavelength division multiplexed (WDM)light including signal lights wavelength division multiplexed together,the amplifier including (a) an optical amplifying medium to Ramanamplify the WDM light in accordance with multiplexed pump lights ofdifferent wavelengths traveling through the optical amplifying medium,the WDM light being amplified in a wavelength band divided into aplurality of individual wavelength bands; and (b) a controllercontrolling output powers of the pump lights in accordance withdifferences in power of the WDM light before being amplified by theoptical amplifying medium and after being amplified by the opticalamplifying medium in each individual wavelength band.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe preferred embodiments, taken in conjunction with the accompanyingdrawings of which:

FIG. 1 is a diagram illustrating the relationship between a pump lightand gain wavelength during Raman amplification, according to anembodiment of the present invention.

FIG. 2 is a diagram illustrating enlargement of bandwidth of a Ramanamplifier by multiplexing different wavelengths of different pump lightsources, according to an embodiment of the present invention.

FIG. 3 is a diagram illustrating a Raman amplifier, according to anembodiment of the present invention.

FIGS. 4(A), 4(B) and 4(C) are diagrams illustrating wavelengthcharacteristics of a single pump light source block of a Ramanamplifier, according to an embodiment of the present invention.

FIGS. 5(A), 5(B) and 5(C) are diagrams illustrating wavelengthcharacteristics of single pump light source block of a Raman amplifier,according to an embodiment of the present invention.

FIGS. 6(A) and 6(B) are diagrams illustrating control to obtain aconstant wavelength characteristic, according to an embodiment of thepresent invention.

FIG. 7 is a flowchart illustrating the operation of a pump lightcontroller in FIG. 3, according to an embodiment of the presentinvention.

FIG. 8 is a diagram illustrating a Raman amplifier, according to anembodiment of the present invention.

FIG. 9 is a diagram illustrating a wavelength characteristic when adesired number of monitor blocks are used in a Raman amplifier,according to an embodiment of the present invention.

FIG. 10 is a diagram illustrating a practical structure of a pump lightsource block and a wavelength multiplexing coupler in the Ramanamplifiers of FIGS. 3 and 8, according to an embodiment of the presentinvention.

FIG. 11 is a diagram illustrating a portion of a Raman amplifier,according to an embodiment of the present invention.

FIG. 12 is a diagram illustrating a Raman amplifier, according to anembodiment of the present invention.

FIG. 13 is a flowchart illustrating the operation of a pump lightcontroller in FIG. 12, according to an embodiment of the presentinvention.

FIG. 14 is a diagram illustrating a Raman amplifier, according to anembodiment of the present invention.

FIG. 15 is a diagram illustrating a Raman amplifier, according to anadditional embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A Raman amplifier is used to compensate for output tilt of an EDFA.

In addition, attention is also paid to a Raman amplifier because thepump light is introduced into the transmission fiber. In this manner,the transmission fiber is used to compensate for deterioration of outputusing the transmission fiber as the Raman amplifying medium, to therebyprovide transmission loss wavelength compensation of the transmissionline due to the induced Raman scattering.

Raman amplifiers can mainly be considered for the following:

(1) Amplification outside of the wavelength band of EDFA.

(2) Improvement in output deviation compensation of an EDFA andimprovement in optical SNR.

(3) Induced Raman scattering compensation of the transmission line.

In a WDM optical communication system, important characteristics for anoptical amplifier are a wideband wavelength band, and a flat wavelengthband.

It is now considered to use a plurality of pump lights of differentwavelengths in view of realizing wide band transmission of a Ramanamplifier. The Raman amplifier output is monitored or an output afterinsertion of an in-line amplifier after the Raman amplifier is monitoredto control outputs of a plurality of pump LDs used to attain the band ofthe Raman amplifier to make small the output deviation.

When three or more pump light sources are used, the algorithms of theoutput power constant control or gain constant control and wavelengthcharacteristic flattening control are extremely complicated.

Namely, with an increase in the number of pump wavelengths for realizingwide band and wavelength flattening or the number of pump light sources,more complicated control algorithms are required. Unfortunately, thereare no conventionally known adequate algorithms.

Reference will now be made in detail to the present preferredembodiments of the present invention, examples of which are illustratedin the accompanying drawings, wherein like reference numerals refer tolike elements throughout.

FIG. 1 is a diagram illustrating the relationship between a pump lightand gain wavelength during Raman amplification, according to anembodiment of the present invention. Referring now to FIG. 1, pumpslights λ_(p1), λ_(p2), and λ_(p3) are pump lights for a Raman amplifier,and have associated Raman shifts of shift1, shift2 and shift 3,respectively. The center wavelength of gain and the gain bandwidth areshifted to a longer wavelength side as much as the shift of pumpwavelength when the pump wavelength is shifted to the longer wavelengthside.

Therefore, a Raman amplifier generates a gain at a respective wavelengththat is shifted in amount of Raman shift of the amplifying medium fromthe pump light wavelength. The Raman shift amount and Raman bandwidthare intrinsically given to a substance (amplifying medium). Thus, Ramanamplification is an optical amplification technique in which gain can beobtained at any desired wavelength if a pump light source having adesired wavelength can be provided.

FIG. 2 is a diagram illustrating enlargement of bandwidth of a Ramanamplifier by multiplexing different wavelengths of different pump lightsources, according to an embodiment of the present invention. Referringnow to FIG. 2, a plurality of pump light sources provide pumps lightswith wavelengths λ_(p1), λ_(p2), and λ_(p3), which together form pumplight 100 applied to an amplifying medium. Wavelengths λ_(p1), λ_(p2),and λ_(p3) are slightly different from each other. In this manner, gain102 providing wideband optical amplification can be realized.

FIG. 3 is a diagram illustrating a Raman amplifier, according to anembodiment of the present invention. Referring now to FIG. 3, the Ramanamplifier includes an input port 0, a Raman amplifying medium 1, amultiplexing coupler 2, a demultiplexing coupler 3, a multiplexingcoupler 4, a wavelength branching coupler 5, pump light source blocks6-1, 6-2 and 6-3, light receiving elements 7-1, 7-2 and 7-3 and a pumplight controller 8.

A wavelength division multiplexed (WDM) light 104 including a pluralityof signal lights multiplexed together is incident to back pumped Ramanamplifying medium 1 from the input port 0.

Multiplexing coupler 4 is a wavelength multiplexing coupler multiplexingthe pump lights of average wavelength of λ_(p1), λ_(p2), and λ_(p3) ofdifferent center wavelengths from pump light source blocks 6-1, 6-2 and6-3, respectively.

Multiplexing coupler 2 is a wavelength multiplexing couplermultiplexing, in Raman amplifying medium 1, the multiplexed pump lightsfrom multiplexing coupler 4 with signal lights traveling through Ramanamplifying medium 1.

Demultiplexing coupler 3 is a light splitter demultiplexing thewavelength-multiplexed light amplified in Raman amplifying medium 1 witha ratio of, for example, 10:1.

Wavelength demultiplexing coupler 5 is a wavelength band demultiplexingcoupler demultiplexing the Raman gain wavelength band generated with thepump light from pump light source blocks 6-1, 6-2 and 6-3 into monitorblocks 1, 2 and 3 (not illustrated in FIG. 3). Each monitor block 1, 2and 3 has a corresponding wavelength band. Light receiving elements 7-1,7-2 and 7-3 receive the wavelength bands, respectively, corresponding tomonitor blocks 1, 2 and 3, respectively, and perform optical/electricconversion.

Pump light controller 8 controls the output powers of averagewavelengths λ_(p1), λ_(p2), and λ_(p3) of pump light source blocks 6-1,6-2 and 6-3 in accordance with the output of the signal light receivingelements 7-1, 7-2 and 7-3.

Control performed by pump light controller 8 will be explained below.

The average pump wavelength of pump light source block 6-1 is defined asλ_(p1), and the output power of the pump light source block 6-1 isdefined as P_(p1). The average pump wavelength of the pump light sourceblock 6-2 is defined as λ_(p2), and the output power of pump lightsource block 6-2 is defined as λ_(p2). The average pump wavelength ofpump light source block 6-3 is defined as λ_(p3), and the output powerof pump light source block 6-3 is defined as P_(p3).

The average output power of the average wavelength λ_(s1) of thewavelength band of the monitor block 1 received with the light receivingelement 7-1 is defined as P_(s1). The average output power of theaverage wavelength λ_(s2) of the wavelength band of the monitor block 2received with the light receiving element 7-2 is defined as P_(s2). Theaverage output power of the average wavelength λ_(s3) of the wavelengthband of the monitor block 3 received with the light receiving element7-3 is defined as P_(s3).

FIGS. 4(A), 4(B) and 4(C) are diagrams illustrating wavelengthcharacteristics of a single pump light source block of a Ramanamplifier, according to an embodiment of the present invention.

More specifically, FIG. 4(A) is a diagram illustrating a wavelengthdivision multiplexed light output from the amplifier when only pumplight source block 6-1 is operated in the average pump wavelength ofλ_(p1), and average pump output power of P_(p1). Referring now to FIG.4(A), a fine solid line 110 indicates the output spectrum while a thicksolid line 112 indicates the average output power of each wavelengthband monitor block by driving only P_(p1).

FIG. 4(B) is a diagram illustrating a wavelength division multiplexlight output from the amplifier when only pump light source block 6-2 isoperated in the average pump wavelength λ_(p2) and average pump outputpower of P_(p2). A fine solid line 114 indicates the output spectrumwhile a thick solid line 116 indicates the average output power of eachwavelength band monitor block by driving only P_(p2).

FIG. 4(C) is a diagram illustrating a wavelength division multiplexlight output from the amplifier when only pump light source block 6-3 isoperated in the average pump wavelength λ_(p3) and average pump outputpower of P_(p3). A fine solid line 118 indicates the output spectrumwhile a thick solid line 120 indicates the average output power of eachwavelength band monitor block by driving only P_(p3).

As can be seen from FIGS. 4(A), 4(B) and 4(C), pump light source block6-1 provides a maximum contribution to the signal light output ofmonitor block 1. Pump light source block 6-2 provides a maximumcontribution to the signal light output of monitor block 2. Pump lightsource block 6-3 provides a maximum contribution to the signal lightoutput of monitor block 3.

Simultaneously, pump light source block 6-1 also makes some contributionto the signal light output of monitor block 2 and the signal lightoutput of monitor block 3. Pump light source block 6-2 makes somecontribution to the signal light output of monitor block 1 and thesignal light output of monitor block 2. Pump light source block 6-3makes some contribution to the signal light output of monitor block 1and signal light output of monitor block 2.

Therefore, pump lights of a plurality of wavelengths can used to form awideband optical amplifier. At least one of the pump lights can becontrolled, and will influence the other wavelength band monitor blocks.

In order to obtain a predetermined amplified signal power, a gaincoefficient is multiplied by the power of a pump light source.Therefore, when the average power variation of the pump light outputs ofthe pump light source blocks 6-1 to 6-3 is defined as ΔP_(p), thevariation of average output power of the band in which the gain isgenerated with the pump lights from the light receiving elements 7-1 to7-3 is defined as ΔP_(s) and the average gain coefficient is defined asA, the following Formula 1 can be determined.ΔP _(s) =A·ΔP _(p)  Formula 1

To eliminate output power wavelength characteristic deviation of eachwavelength block, ΔP_(p) can be adjusted to make identical the powerlevels of the wavelength-multiplex signal lights of wavelength bandsdemultiplexed into three bands with the wavelength demultiplexingcoupler 5. ΔP_(p) can be adjusted, for example, by varying an opticaloutput power of the pump light source, by varying the pump wavelength toshift the center of gravity wavelength and also by varying the pumplight wavelength width. Here, an example of adjustment for varying anoptical output power will be explained.

As illustrated in FIGS. 4(A), 4(B) and 4(C), since the gain wavelengthband generated by one pump light source block is wide and the gain isgenerated over each monitor block, when one pump light source block isvaried, Formula 1 must be calculated, considering the influence on thewavelength of the other monitor blocks.

In other words, regarding the power of each monitor block, an outputpower of each pump light source block should be controlled based on thewavelength characteristic of the gain generated in the opticalamplifying medium of each pump light source block.

Here, the average gain coefficient of the average output power variationΔP_(p1) of the pump wavelength λ_(p1) of the pump light source block 6-1affecting on the average output power variation ΔP_(s1) of the monitorblock 1 is defined as A₁₁. The average gain coefficient of the averageoutput power variation ΔP_(p1) of the pump wavelength λ_(p1) of the pumplight source block 6-1 affecting on the average output power variationΔP_(p1) of the monitor block 2 is defined as A₂₁. The average gaincoefficient of the average output power variation ΔP_(p1) of the pumpwavelength λ_(p1) of the pump light source block 6-1 affecting on theaverage output power variation ΔP_(s3) of the monitor block 3 is definedas A₃₁.

The average gain coefficient of the average output power variationΔP_(p2) of the pump wavelength λ_(p2) of the pump light source block 6-2affecting on the average output power variation ΔP_(s1) of the block 1of the monitor block is defined as A₁₂. The average gain coefficient ofthe average output power variation ΔP_(p2) of the pump wavelength λ_(p2)of the pump light source block 6-2 affecting on the average output powervariation ΔP_(s2) of the monitor block 2 is defined as A₂₂. The averagegain coefficient of the average output power variation ΔP_(p2) of thepump wavelength λ_(p2) of the pump light source block 6-2 affecting onthe average output power variation ΔP_(s3) of the monitor block 3 isdefined as A₃₂.

The average gain coefficient of the average output power variationΔP_(p3) of the pump wavelength λ_(p3) of the pump light source block 6-3affecting on the average output power variation ΔP_(s1) of the monitorblock 1 is defined as A₁₃. The average gain coefficient of the averageoutput power variation ΔP_(p3) of the pump wavelength λ_(p3) of the pumplight source block 6-3 affecting on the average output power variationΔP_(s2) of the monitor block 2 is defined as A₂₃. The average gaincoefficient of the average output power variation ΔP_(p3) of the pumpwavelength λ_(p3) of the pump light source block 6-3 affecting on theaverage output power variation ΔP_(s3) of the monitor block 3 is definedas A₃₃.

FIGS. 5(A), 5(B) and 5(C) are diagrams illustrating wavelengthcharacteristics of a single pump light source block of a Ramanamplifier, according to an embodiment of the present invention.

More specifically, FIG. 5(A) illustrates the average output powerdifference of the monitor block 1, the monitor block 2 and the monitorblock 3 for the pump light output power difference when only the pumplight source block 6-1 is operated. Respective gradients correspond toA₁₁, A₂₁, A₃₁.

FIG. 5(B) illustrates the average output power difference of the monitorblock 1, the monitor block 2 and the monitor block 3 for the pump lightoutput power difference when only the pump light source block 6-2 isoperated. Respective gradients correspond to A₁₂, A₂₂, A₃₂.

FIG. 5(C) illustrates the average output power difference of the monitorblock 1, the monitor block 2 and the monitor block 3 for the pump lightoutput power difference when only the pump light source block 6-3 isoperated. Respective gradients correspond to A₁₃, A₂₃, A₃₃.

Here, the average gain coefficient matrix [A] including these elementscan be obtained.

$\begin{matrix}{\begin{bmatrix}{\Delta\; P_{S1}} \\{\Delta\; P_{S2}} \\{\Delta\; P_{S3}}\end{bmatrix} = {\begin{bmatrix}A_{11} & A_{12} & A_{13} \\A_{21} & A_{22} & A_{23} \\A_{31} & A_{32} & A_{33}\end{bmatrix}\;\begin{bmatrix}{\Delta\; P_{P1}} \\{\Delta\; P_{P2}} \\{\Delta\; P_{P3}}\end{bmatrix}}} & {{Formula}\mspace{20mu} 2}\end{matrix}$

FIGS. 6(A) and 6(B) are diagrams illustrating control to obtain aconstant wavelength characteristic, according to an embodiment of thepresent invention.

Referring now to FIG. 6(A), the average output of the monitor block 1,the monitor block 2 and the monitor block 3 when the wavelengthcharacteristic of the signal light output has a large signal lightspectrum is indicated with a thick solid line 122 and the average outputP_(f) of the total wavelength band is indicated with a broken line 124.

Reduction of the wavelength characteristic deviation of the wavelengthmultiplex light output indicates that the average outputs P_(s1), P_(s2)and P_(s3) of monitor blocks 1, 2 and 3, respectively, are matched, asillustrated in FIG. 6(B), with the target Raman-amplified wavelengthmultiplex light output P_(f) (average output of total wavelength band).ΔP _(s1) =|P _(f) −P _(s1)|ΔP _(s2) =|P _(f) −P _(s2)|ΔP _(s3) =|P _(f) −P _(s3)|  Formula 3ΔP _(s1) ≈ΔP _(s2) =ΔP _(s3)  Formula 4

Output difference (tilt) can be suppressed small in the total wavelengthband where the Raman gain is generated in the Raman amplifying medium 1by calculating the compensation amount of the pump light outputs P_(p1),P_(p2) an P_(p3) of the pump light source blocks 6-1, 6-2 and 6-3,respectively, to satisfy the above formula.

$\begin{matrix}{\begin{bmatrix}{\Delta\; P_{P1}} \\{\Delta\; P_{P2}} \\{\Delta\; P_{P3}}\end{bmatrix} = {\begin{bmatrix}A_{11} & A_{12} & A_{13} \\A_{21} & A_{22} & A_{23} \\A_{31} & A_{32} & A_{33}\end{bmatrix}^{- 1}\begin{bmatrix}{\Delta\; P_{S1}} \\{\Delta\; P_{S2}} \\{\Delta\; P_{S3}}\end{bmatrix}}} & {{Formula}\mspace{20mu} 5}\end{matrix}$

Namely, it is enough for the pump light controller 8 of FIG. 3 tocontrol the pump light power output from each pump light source block6-1, 6-2 and 6-3 by (a) monitoring the output power by dividing thewavelength-multiplex light where a plurality of signal lights arewavelength-multiplexed into the monitor blocks of the predeterminedwavelength band, (b) executing the average value process obtained bydividing total output of the monitor block of each wavelength band withthe number of channels, and (c) calculating, with the Formula 5, theaverage output power difference (tilt) of the pump light for weightingthe influence on the wavelength of each monitor block of the pumpwavelength of each pump light source block required to reduce the outputpower difference in the total wavelength band.

Moreover, the feedback control might typically be performed, forexample, up to about ten (10) times until the predetermined wavelengthcharacteristic deviation is obtained.

With these control processes, the average power of the Raman gainwavelength band generated with the pump light can be set to the constantpower P_(f).

FIG. 7 is a flowchart illustrating a process performed by pump lightcontroller 8 in FIG. 3, according to an embodiment of the presentinvention. The processes in FIG. 7 can be performed, for example, such aprocessor, such as a CPU.

Referring now to FIG. 7, in operation 1, the control process is started.

From operation 1, the process moves to operation 2, where the averageoutput powers P_(s1), P_(s2) and P_(s3) in the monitor blocks 1, 2 and3, respectively, are obtained from the outputs of the light receivingelements 7-1, 7-2 and 7-3, respectively.

From operation 2, the process moves to operation 3, where ΔP_(s1),ΔP_(p2) and ΔP_(s3) are obtained by comparing the average wavelengthoutput powers P_(s1), P_(s2), and P_(s3) in the monitor block 1, 2 and3, respectively, with the target wavelength multiplex output valueP_(f).

From operation 3, the process moves to operation 4, where it isdetermined whether the difference between ΔP_(s1) to ΔP_(s3) and P_(f)is within an allowable range. If the difference is within the allowablerange, the process moves to operation 7 where the process stops. If thedifference is not within the allowable range, the process moves tooperation 5, where control amounts ΔP_(p1), ΔP_(p2) and ΔP_(p3) of thepower levels P_(p1), P_(p2) and P_(p3) of the pump light source blocks6-1, 6-2 and 6-3 are obtained, from ΔP_(s1), ΔP_(s2), ΔP_(p3), using theinverse matrix of the average gain coefficients A₁₁ to A₃₃ which areaffected on each monitor block by each pump light.

From operation 5, the process moves to operation 6, where the outputpowers P_(p1), P_(p2) and P_(p3) of the pump light source blocks 6-1,6-2, 6-3, respectively, are controlled by adding the control amountsΔP_(p1), ΔP_(p2), ΔP_(p3) to the current P_(p1), P_(p2), P_(p3),respectively.

From operation 6, the process moves to operation 7, where controlprocess is completed.

In FIG. 3, as an example, a total pump light source block is provided asthe three pump light source blocks 6-1, 6-2 and 6-3, and the totalmonitor block of the wavelength band that generates the gain through thepump light from the pump light source block is divided into threemonitor blocks. However, the present invention is not limited to a totalpump light source block provided “three” pump light source blocks, or atotal monitor block as divided into “three” monitor blocks. Instead, thenumber of pump light source blocks of the total pump light source blockand the number of monitor blocks of the total monitor block can be setto any practical number, which would typically be a matter of designchoice.

For example, FIG. 8 is a diagram illustrating a Raman amplifier,according to an additional embodiment of the present invention. In FIG.8, the number of pump light source blocks and monitor blocks can be setfreely. Thus, in FIG. 8, n pump light source blocks (6-1 to 6-n) and mmonitor blocks of the wavelength multiplex signal light are provided.

FIG. 9 is a diagram illustrating a wavelength characteristic when adesired number of monitor blocks are used in a Raman amplifier,according to an embodiment of the present invention. More specifically,FIG. 9 illustrates the wavelength band of the Raman amplification gainfor the wavelength division multiplexed light decoupled by wavelengthdemultiplex coupler 5 in FIG. 8, with the wavelength band being dividedinto m monitor blocks.

Variation ΔP_(p) of the pump light power control is expressed as an n×1matrix. Difference ΔP_(s) between the average value of the wavelengthmultiplex signal light power in the monitor block and the target controlvalue is expressed as the m×1 matrix. A is expressed as the n×m matrix.

$\begin{matrix}{\begin{bmatrix}{\Delta\; P_{S_{1}}} \\{\Delta\; P_{S_{2}}} \\\bullet \\\bullet \\{\Delta\; P_{S_{n}}}\end{bmatrix} = {\begin{bmatrix}A_{11} & A_{12} & \bullet & \bullet & A_{1m} \\A_{21} & A_{22} & \bullet & \bullet & A_{2m} \\\bullet & \bullet & \bullet & \bullet & \bullet \\\bullet & \bullet & \bullet & \bullet & \bullet \\A_{n1} & A_{n2} & \bullet & \bullet & A_{nm}\end{bmatrix}\;\begin{bmatrix}{\Delta\; P_{P_{1}}} \\{\Delta\; P_{P_{2}}} \\\bullet \\\bullet \\{\Delta\; P_{P_{M}}}\end{bmatrix}}} & {{Formula}\mspace{20mu} 6}\end{matrix}$

ΔP_(pi), in this case, is variation of the average output power of thepump light source, while Δp_(sj) is variation of the average outputpower of the signal light monitor block.

Since variation is used, it is not required to convert the monitoroutput power to the main signal output power.

Here, it is understood that ΔP_(pi) resulting from ΔP_(sj) can beobtained by obtaining the inverse matrix [A]⁻¹ of [A].

$\begin{matrix}{\begin{bmatrix}{\Delta\; P_{P_{1}}} \\{\Delta\; P_{P_{2}}} \\\bullet \\\bullet \\{\Delta\; P_{P_{m}}}\end{bmatrix} = {A^{- 1}\begin{bmatrix}{\Delta\; P_{S_{1}}} \\{\Delta\; P_{S_{2}}} \\\bullet \\\bullet \\{\Delta\; P_{S_{n}}}\end{bmatrix}}} & {{Formula}\mspace{20mu} 7}\end{matrix}$

Therefore, reduction of deviation of the average output power among eachblock indicates flattening of the wavelength characteristic of thesignal light output power.

In the embodiment of FIG. 3, the desired number of pump light sourceblocks and monitor blocks is not limited to any particular number andcan be determined in accordance with design choice. However, it ispreferable that the number of monitor blocks be less than the number ofsignal light channels multiplexed to the wavelength multiplex light, andexceeding the number of pump light source blocks.

FIG. 10 is a diagram illustrating a practical structure of a pump lightsource block and a wavelength multiplexing coupler in the Ramanamplifiers of, for example, FIGS. 3 and 8, according to an embodiment ofthe present invention. Referring now to FIG. 10, the embodiment includesWDM couplers 24 and 25, deflection composite couplers 61, 62 and 63,fiber grating filters 51, 52, 53, 54, 55 and 56, and semiconductorlasers 81, 82, 83, 84, 85 and 86.

The pump light source block 6-1 includes semiconductor lasers 81 and 82.The pump light source block 6-2 includes semiconductor lasers 83 and 84.The pump light source block 6-3 includes semiconductor lasers 85 and 86.Semiconductor lasers 81 and 82 have slightly different wavelengths.Semiconductor lasers 83 and 84 have slightly different wavelengths.Semiconductor lasers 85 and 86 have slightly different wavelengths. Inthe example of FIG. 10, the various pairs of semiconductor lasers havewavelengths which are about 4 nm apart, but the present invention is notlimited to this specific wavelength difference.

The pump lights from the semiconductor lasers 81 and 82 are at, forexample, wavelengths 1429.7 nm and 1433.7 nm, respectively, and arereflected at the fiber grating filters 51 and 52, respectively, toprovide a resonance structure to output a pump light of the particularwavelength. PBS coupler 61 multiplexes these pump lights, to provide apump light provided by pump light source block 6-1.

The pump lights from the semiconductor lasers 83 and 84 are at, forexample, wavelengths 1454.0 nm and 1458.0 nm, respectively, and arereflected at the fiber grating filters 53 and 54, respectively, toprovide a resonance structure to output a pump light of the particularwavelength. PBS coupler 62 multiplexes these pump lights, to provide apump light provided by pump light source block 6-2.

The pump lights from the semiconductor lasers 85 and 86 at, for example,wavelengths 1484.5 nm and 1488.5 nm, respectively, and are reflected atthe fiber grating filters 55 and 56, respectively, to provide aresonance structure to output a pump light of the particular wavelength.PBS coupler 63 multiplexes these pump lights, to provide a pump lightprovided by pump light source block 6-3.

The polarization coupling by PBS couplers 61, 62 and 63 is performed,for example, to eliminate dependence on change of the Ramanamplification.

The multiplex coupler 4 includes the WDM couplers 24 and 25. The WDMcoupler 25 operates, for example, by reflecting the wavelength lightfrom the pump light source block 6-2 and transferring the wavelengthfrom the pump light source block 6-3. The WDM coupler 24 operates, forexample, by reflecting the wavelength light from the pump light sourceblock 6-1 and transferring the wavelength from the pump light sourceblock 6-3.

In FIG. 10, in each pump light source block 6-1, 6-2 and 6-3, thevarious semiconductor laser-fiber grating pairs output light which isslightly different in wavelength from each other. However, the presentinvention is not limited to this, and equal wavelength can be output.Moreover, the light of each pump light source block is not required tobe formed with a plurality of semiconductor lasers. For example, a pumplight of a pump light source block can be formed by a single lightsource which does not depend on polarization.

In FIG. 3, the target wavelength multiplex light output value is definedas P_(f) and the average powers of all wavelength bands are controlledto become equal to P_(f). Therefore, it is possible to perform controlto obtain constant output in all wavelength bands.

As a modified example of this constant output control, P_(f) is definedas P_(f1), P_(f2), P_(f3) for each wavelength band, or monitor block, ofthe total monitor block and these values are compared to conductindividual constant output control in the individual monitor blocks.

In this case, P_(f1), P_(f2), P_(f3) correspond to monitor blocks 1, 2and 3, respectively, in place of P_(f) in operation 4 of the flowchartof FIG. 7.

The pump light controller 8 may also be controlled by subtracting thecorresponding P_(s1), P_(s2), P_(s3) from the values P_(f1), P_(f2),P_(f3).

FIG. 11 is a diagram illustrating a portion of a Raman amplifier,according to an embodiment of the present invention. Referring now toFIG. 11, weighting can be performed freely in monitor blocks 1, 2 and 3to conduct constant output control individually in monitor blocks 1, 2and 3, by providing, in place of changing P_(f) variable or fixedattenuators 71, 72 and 73 in the preceding stage of the light receivingelements 7-1, 7-2 and 7-3 of FIG. 3.

Moreover, the embodiment in FIG. 3 can freely use, as the Ramanamplifying medium, for example, dispersion compensation fiber (DCF)resulting in small effective sectional area and large non-linearity,dispersion shifted fiber (DSF) and non-zero dispersion shifted fiber(NZDSF), as well as the ordinary 1.3 zero-micron fiber.

When fibers having large non-linearity are used, the length of the fiberthat operates as the Raman amplifying medium to obtain the necessarygain can be shortened. Therefore, centralized amplification can berealized.

In the embodiment of FIG. 3, the wavelength demultiplex couplers 3 and5, and light receiving elements 7-1, 7-2 and 7-3, are used to provide amonitor block. Instead, however, a spectrum analyzer can be used.

FIG. 12 is a diagram illustrating a Raman amplifier, according to anadditional embodiment of the present invention. In FIG. 12, a branchingcoupler 9, a wavelength-demultiplexing coupler 10 and light receivingelements 11-1, 11-2 and 11-3 are also used to provide a monitor block,in addition to elements of FIG. 3.

In FIG. 12, a plurality of wavelength-multiplexed signals are providedthe input-port 0 of the Raman amplifier. The branching coupler 9 is alight splitter provided at the input port 0 to branch thewavelength-multiplexed signals by, for example, a 10:1 ratio.

The wavelength demultiplexing coupler 10 is a wavelength band branchingcoupler for dividing the Raman gain wavelength band generated from thepump light transmitted from the pump light source blocks 6-1, 6-2 and6-3 into the three wavelength bands (monitor blocks), in a similarmanner as the wavelength demultiplexing coupler 5. Namely, wavelengthdemultiplexing coupler 10 is a wavelength demultiplexing filter fordemultiplex the Raman gain wavelength band into monitor blocks 1, 2 and3 of the wavelength band.

The light receiving elements 11-1, 11-2 and 11-3 convert the opticalpower of the monitor blocks 1, 2 and 3, respectively.

Regarding monitor blocks 1, 2 and 3 isolated by the wavelengthdemultipexing coupler 10, the average output power of the averagewavelength λ_(s1) of the monitor block 1 is defined as P_(in) _(—)_(s1), the average output power of the average wavelength λ_(s2) of themonitor block 2 is defined as P_(in) _(—) _(s2), and the average outputpower of the average wavelength λ_(s3) of the monitor block 3 is definedas P_(in) _(—) _(s3).

The main signal light is incident to the back pumped Raman amplifyingmedium 1.

The pump light source blocks 6-1, 6-2 and 6-3 may be constructed, forexample, as illustrated in FIG. 10 or may be realized in variousembodiments like that for the embodiment in FIG. 3.

The signal amplified with the amplifying medium 1 is branched withbranching coupler 3 by, for example, a 10:1 ratio, and divided into thethree wavelength band blocks like that of the wavelength demultiplexingcoupler 10.

The wavelength band of the wavelength demultiplexing coupler 5respectively corresponds to the average wavelengths λ_(s1), λ_(s2),λ_(s3) of the monitor block of the wavelength branching coupler 10. Thewavelength multiplex output power is photo-electrically converted in thelight receiving elements 7-1, 7-2 and 7-3.

As with FIG. 3, the average output power of the average wavelengthλ_(s1) of the monitor block 1 of the wavelength demultipexing coupler 5is defined as P_(s1), the average output power of the average wavelengthλ_(s2) of the monitor block 2 is defined as P_(s1), and the averageoutput power of the average wavelength λ_(s3) of the monitor block 3 isdefined as P_(s3).

The pump light controller 8 controls the gain to a predetermined valuewith the monitor input from the light receiving elements 7-1, 7-2, 7-3,11-1, 11-2 and 11-3.

Practical operations of the pump light controller 8 in FIG. 12 areexplained below.

The average gains G₁, G₂, G₃ of monitor blocks 1, 2 and 3, respectively,can be obtained by subtracting P_(in) _(—) _(s1), P_(in) _(—) _(s2),P_(in) _(—) _(s3) obtained with the light receiving elements 11-1, 11-2and 11-3 through isolation with the wavelength demultiplexing coupler 10in the input port side from P_(s1), P_(s2), P_(s3) obtained with thelight receiving elements 7-1, 7-2 and 7-3, respectivelyG ₁ =P _(s1) −P _(in) _(—) _(s1)G ₂ =P _(s2) −P _(in) _(—) _(s2)G ₃ =P _(s3) −P _(in) _(—) _(s3)  Formula 8

The pump light average output power of each monitor block and thewavelength light average gain of each monitor block may be coupled withthe average gain coefficient of each monitor block and when the pumplight average output power variation amount is ΔP_(p), the signal lightaverage output power variation amount is ΔG, and the average gaincoefficient is A.ΔG=A·ΔP _(p)  Formula 9

[A] used in the embodiment for FIG. 3 indicates gradient of the signallight average output power of the pump light average output power.Therefore, the following relationship can also be established for thegain A defined here.

$\begin{matrix}{\begin{bmatrix}{\Delta\; G_{1}} \\{\Delta\; G_{2}} \\{\Delta\; G_{3}}\end{bmatrix} = {\begin{bmatrix}A_{11} & A_{12} & A_{13} \\A_{21} & A_{22} & A_{23} \\A_{31} & A_{32} & A_{33}\end{bmatrix}\;\begin{bmatrix}{\Delta\; P_{p1}} \\{\Delta\; P_{p2}} \\{\Delta\; P_{p3}}\end{bmatrix}}} & {{Formula}\mspace{20mu} 10}\end{matrix}$

Here, the target gain level is defined as average gain G_(f) of thetotal wavelength band, the average gain of each monitor block is definedas G₁, G₂, G₃, the difference of G_(f) and G₁ is defined as ΔG₁, thedifference of G_(f) and G₂ as ΔG₂ and the difference of G_(f) and G₃ asΔG₃.ΔG ₁ =|G _(f) −G ₁|ΔG ₂ =|G _(f) −G ₂|ΔG ₃ =|G _(f) −G ₃|  Formula 11

In order to make small the gain wavelength deviation (tilt) in the totalwavelength band, the average gain among monitor blocks is set equally tomatch with the average gain G_(f) of the total wavelength band.

Here, all wavelengths can be controlled to the constant gain by settingG_(f) to the predetermined value for obtaining the constant gain.ΔG ₁ ≈ΔG ₂ ≈ΔG ₃  formula 12

Therefore, it is possible to calculate ΔP_(p1), ΔP_(p2), ΔP_(p3) fromthe Formula 13 using the Formula 11.

$\begin{matrix}{\begin{bmatrix}{\Delta\; P_{p1}} \\{\Delta\; P_{p2}} \\{\Delta\; P_{p3}}\end{bmatrix} = {\begin{bmatrix}A_{11} & A_{12} & A_{13} \\A_{21} & A_{22} & A_{23} \\A_{31} & A_{32} & A_{33}\end{bmatrix}^{- 1}\begin{bmatrix}{\Delta\; G_{1}} \\{\Delta\; G_{2}} \\{\Delta\; G_{3}}\end{bmatrix}}} & {{Formula}\mspace{20mu} 13}\end{matrix}$

Namely, the pump light controller 8 obtains total output of the monitorblock of the wavelength multiplex light, executes the process to obtainthe average value by dividing total output of the monitor block with thenumber of channels and controls the pump light source blocks of themonitor block by calculating the necessary average output difference ofpump light considering the influence of the gain by each pump lightsource block on the wavelength of each monitor block in view of makingsmall the gain difference in the total wavelength band.

The feedback controls are repeated, for example, up to ten (10) timesuntil the wavelength characteristic deviation (tilt) of the gain of eachmonitor block of the Raman optical amplifier is eliminated.

FIG. 13 is a flowchart illustrating the operation of the pump lightcontroller 8 in FIG. 12, according to an embodiment of the presentinvention. Referring now to FIG. 13, in operation 1, the control isstarted.

From operation 1, the process moves to operation 2, where the gains G₁,G₂ and G₃ of the monitor block are obtained, respectively, bysubtracting the powers P_(in) _(—) _(s1), P_(in) _(—) _(s2), P_(in) _(—)_(s3) of the monitor blocks of the wavelength demultiplexing coupler 5provided in the input side from the powers P_(s1), P_(s2) and P_(s3),respectively, of the monitor blocks of the wavelength demultiplexingcoupler 5 provided in the output side of the optical amplifying medium1.

From operation 2, the process moves to operation 3, where the targetgain G_(f) is compared with the gains G₁, G₂ and G₃ in the monitorblocks to obtain the differences.

From operation 3, the process moves to operation 4, where the differencebetween ΔG₁, ΔG₂ and G_(f) is determined. When difference is within anallowable range in operation 4, the process moves to operation 7, wherethe process stops. When difference is not within the allowable range inoperation 4, the process moves to operation 5.

In operation 5, control amounts ΔP_(p1), ΔP_(p2) and ΔP_(p3) of thepower levels Pp1, Pp2 and Pp3, respectively, of the pump light sourceblocks λ_(p1), λ_(p2) and λ_(p3), respectively, are obtained from ΔG₁,ΔG₂ and ΔG₃ using the average gain coefficients A₁₁ to A₃₃ which affectson each monitor block with each pump light.

From operation 5, the process moves to operation 6, where the outputpowers P_(p1), P_(p2) and P_(p3) of the pump light source blocks 6-1,6-2 and 6-3, respectively, are controlled by adding the control amountsΔP_(p1), ΔP_(p2) and ΔP_(p3) to the current P_(p1), P_(p2) and P_(p3),respectively.

With the flow explained above, the pump light controller 8 controls theindividual pump light source blocks. In the embodiment of FIG. 12, likethe embodiment of FIG. 3, the number of pump light source blocks andmonitor blocks may be set freely.

Namely, when the number of pump light source blocks is set to n, whilethe number of monitor blocks is set to m, the Formula 10, Formula 11,Formula 12 and Formula 13 may be updated as follows.

$\begin{matrix}\begin{matrix}{G_{1} = {P_{s1} - P_{in\_ s1}}} \\{G_{2} = {P_{s2} - P_{in\_ s2}}} \\\vdots \\{G_{m} = {P_{sm} - P_{in\_ sm}}}\end{matrix} & {{Formula}\mspace{20mu} 14} \\{\begin{pmatrix}{\Delta\; G_{1}} \\{\Delta\; G_{2}} \\\bullet \\\bullet \\{\Delta\; G_{m}}\end{pmatrix} = {\begin{bmatrix}A_{11} & A_{12} & \bullet & \bullet & A_{1n} \\A_{12} & A_{22} & \bullet & \bullet & A_{2n} \\\bullet & \bullet & \bullet & \bullet & \bullet \\\bullet & \bullet & \bullet & \bullet & \bullet \\A_{m1} & A_{m2} & \bullet & \bullet & A_{mn}\end{bmatrix}\;\begin{bmatrix}{\Delta\; P_{P_{1}}} \\{\Delta\; P_{P_{2}}} \\\bullet \\\bullet \\{\Delta\; P_{P_{n}}}\end{bmatrix}}} & {{Formula}\mspace{20mu} 15} \\\begin{matrix}{{\Delta\; G_{1}} = {{G_{f} - G_{1}}}} \\{{\Delta\; G_{2}} = {{G_{f} - G_{2}}}} \\\vdots \\{{\Delta\; G_{m}} = {{G_{f} - P_{m}}}}\end{matrix} & {{Formula}\mspace{20mu} 16} \\{{\Delta\; G_{1}} \approx {\Delta\; G_{2}} \approx {\Delta\; G_{m}}} & {{Formula}\mspace{20mu} 17} \\{\begin{bmatrix}{\Delta\; P_{P_{1}}} \\{\Delta\; P_{P_{2}}} \\\bullet \\\bullet \\{\Delta\; P_{pn}}\end{bmatrix} = {A^{- 1}\begin{bmatrix}{\Delta\; G_{1}} \\{\Delta\; G_{2}} \\\bullet \\\bullet \\{\Delta\; G_{m}}\end{bmatrix}}} & {{Formula}\mspace{20mu} 18}\end{matrix}$

Thus, the pump light controller 8 could be designed in accordance withthe above formula.

In the embodiment of FIG. 12, the number of pump light source blocks andmonitor blocks can be set freely as in the case of the embodiment ofFIG. 3, but it is preferable that the number of monitor blocks is setless than the number of signal light channels multiplexed in thewavelength multiplex signal, and exceeding the number of pump lightsource blocks.

Moreover, as with the embodiment of FIG. 3, the embodiment of FIG. 12can freely use, as the Raman amplifying medium, a dispersioncompensation fiber (DCF) resulting in small effective sectional area andlarge non-linearity, a dispersion shift fiber (DSF) and a non-zerodispersion shift fiber (NZDSF) as well as the ordinary 1.3 zero-micronfiber.

When an optical fiber operating as the Raman amplifying medium 1 has alarge non-linearity, the fiber can be relatively short in length, whileproviding centralized amplification.

Moreover, when an optical fiber operating as the Raman amplifying medium1 has a small effective cross-sectional area and intensivenon-linearity, the Raman amplifying medium 1 can be structured in shortlength. However, when an ordinary 1.3 μm zero-discrete fiber is used, alength of about 40 km or longer will probably be required depending onthe pump power.

FIG. 14 is a diagram illustrating a Raman amplifier, according to afurther embodiment of the present invention. More specifically, FIG. 14illustrates an example where input of the wavelength multiplex light ofthe Raman amplifier of FIG. 12 is notified using the actual transmissionline.

Referring now to FIG. 14, a monitor controller (OSC) 12 detects thepower of each monitor block and transmits information of the result tothe Raman amplifying medium 1 as the transmission line via amultiplexing coupler 13 in the wavelength of λ_(osc). The signal ofwavelength λ_(osc) is demultiplexed with the wavelength demultiplexingcoupler 5 and detected with a monitor controller (OSC) 14 and is thensupplied to the pump optical controller 8.

In FIG. 14, the wavelength λ_(osc) is demultiplexed with the wavelengthdemultiplexing coupler 5, but it is possible to additionally provide abranching coupler to the transmission line and to branch the monitorcontrol signal and input this signal to the monitor controller 14.

In FIG. 13, the gain can be kept constant even when the Raman amplifyingmedium 1 is used in the transmission line through the embodimentexplained above in the gain wavelength band of the amplifying medium 1with the pump light from the pump light source block using the samevalue of G_(f) for all monitor blocks. The gain weighted for eachwavelength band of each monitor block can be controlled to the constantvalue by setting the other gain G_(f) for each monitor block.

In addition, as with the embodiment in FIG. 3, with the embodiment inFIG. 12, the weighting process can be performed constantly to G_(f) forall wavelength blocks and it is also possible to conduct the weightingprocess by providing variable or fixed optical attenuators 71 to 73 inthe preceding stage of the light receiving elements in unit of monitorblock.

In the embodiment of FIG. 12, the monitor block is formed via thewavelength demultiplexing couplers 5 and 10, and light receivingelements 7-1, 7-2, 7-3, 11-1, 11-2 and 11-3, but these may be replacedwith the spectrum analyzers.

The embodiments in FIGS. 3 and 13 can be combined with an opticalamplifier using a rare-earth doped fiber (for example, an erbium-dopedfiber).

For example, FIG. 15 is a diagram illustrating a Raman amplifier,according to an additional embodiment of the present invention.Referring now to FIG. 15, a first rare-earth doped fiber amplifier 13-1,a second rare-earth doped fiber amplifier 13-2, a wavelength banddemultiplexing coupler 5-1, branching couplers 5-2, 5-3, 54 and 5-5, afirst wavelength band monitor 5-6, a second wavelength band monitor 5-7,a first spectrum analyzer 5-8 and a second spectrum analyzer 5-9 areprovided.

The wavelength band demultiplexing coupler 5-1 divides the wavelengthmultiplex light amplified with the Raman amplifying medium 1 to a firstwavelength band (C-band: 1530 nm to 1557 nm) and a second wavelengthband (L-band: 1570 nm to 1610 nm) and then outputs these dividedwavelength bands.

The first rare-earth doped fiber amplifier 13-1 is an optical amplifierformed of an erbium-doped fiber (EDF) having the gain for the firstwavelength band. The second rare-earth doped fiber amplifier 13-2 is anoptical amplifier formed of an erbium-doped fiber (EDF) having the gainfor the second wavelength band.

One light branched with the wavelength band demultiplexing coupler 5-1is amplified by the first rare-earth doped fiber amplifier in the firstwavelength band, and the other light branched with wavelength banddemultiplexing coupler 5-1 is amplified by the second rare-earth dopedfiber amplifier in the second wavelength band.

The branching couplers 5-2, 5-3 are branching couplers for branching thelight of the first wavelength band in the ratio of, for example, 10:1.The branching couplers 5-4, 5-5 are branching couplers for branching thelight of the second wavelength band in the ratio of, for example, 10:1.

The first wavelength band monitor 5-6 monitors the power of the firstwavelength band light branched with the branching coupler 5-2. Thesecond wavelength band monitor 5-7 monitors the power of the secondwavelength band light branched with the branching coupler 5-4.

The pump light controller 8 calibrates the output powers of the firstspectrum analyzer 5-8 and second spectrum analyzer 5-9 based on theoutputs of the first and second wavelength monitors 5-6 and 5-7. Outputsof the spectrum analyzers 5-8 and 5-9 are divided to the wavelength bandblocks of, for example, 1528.773 to 1552.122 nm, 1552.524 to 1563.455nm, 1570.416 to 1581.601 nm, and 1582.018 to 1607.035 nm, to obtain theaverage output of each monitor block in view of controlling the pumplights 6-1, 6-2 and 6-3.

In the embodiment of FIG. 12, it is possible to use an output of thewavelength demultiplexing coupler 10 of FIG. 12 and FIG. 14 and use amethod for detecting the signal before Raman amplification from themonitor controller 14 using the monitor control wavelength signal OSC.

According to the above embodiments of the present invention, a pluralityof pump light sources are used to realize a wideband Raman amplifierwith flattening of the wavelength characteristic of output and gain. Thepresent invention enables control of wavelength characteristic deviationof output power and gain, control of constant output, and control ofconstant gain using a simplified control algorithm. In variousembodiments of the present invention, the number of wavelength bands formonitoring an amplified light are higher than the number of individualblocks forming a pump light source block and lower than the number ofsignal channels.

In the various examples provided herein, specific wavelengths,frequencies and other values are provided for explanation purposes.However, the present invention is not limited to such specificwavelengths, frequencies or other values.

Although a few preferred embodiments of the present invention have beenshown and described, it would be appreciated by those skilled in the artthat changes may be made in these embodiments without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

1. An apparatus comprising: optical amplifier for amplifying awavelength division multiplexed (WDM) light including signal lightswavelength division multiplexed together, the amplifier comprising anoptical fiber through which the WDM light travels and is therebyamplified via Raman amplification in accordance with multiplexed pumplights of different wavelengths traveling through the optical fiber; anoptical device dividing the amplified WDM light into first and seconddivided lights in first and second wavelength bands, respectively; afirst optical amplifier amplifying the first divided light, the firstoptical amplifier having a gain band including the first wavelengthband; a second optical amplifier amplifying the second divided light,the second optical amplifier having a gain band including the secondwavelength band; and means for controlling power of each pump lightbased on a power of the first divided light amplified by the firstoptical amplifier and a power of the second divided light amplified bythe second optical amplifier to maintain a substantially flat gain overthe first and second wavelength bands.